It is known in such systems as gyroscopes and radar antennas to mount a sensor in a frame in inner and outer gimbal rings, to enable rotation of the sensor about each of a pair of mutually orthogonal axes. Angulation of the outer ring with respect to the frame is straight forward, but angulation of the inner ring is conventionally measured and produced with respect to the outer ring. This has two disadvantages. First, space for the measuring and actuating mechanisms of the inner gimbal must be furnished within one or both gimbal rings, thus reducing the size of sensors which can be installed in any such unit. Second, the angulation of the inner ring with respect to the frame may not be obtained directly, but must be determined by a somewhat cumbersome mathematical computation.